The present invention relates to piezoelectric devices whose active material is a superlattice material.
A piezoelectric material is one that exhibits the piezoelectric effect. Piezoelectricity is electricity, or electric polarity, resulting from the application of mechanical pressure on a dielectric crystal. It has long been known that the application of a mechanical stress produces in certain dielectric (electrically nonconducting) crystals an electric polarization (electric dipole) moment per cubic meter which is proportional to this stress. If the crystal is isolated, this polarization manifests itself as a voltage across the crystal, and if the crystal is short-circuited, a flow of charge can be observed during loading. Conversely, application of a voltage between certain faces of the crystal produces a mechanical distortion of the material. This reciprocal relationship is referred to as the piezoelectric effect. The phenomenon of generation of a voltage under mechanical stress is referred to as the direct piezoelectric effect, and the mechanical strain produced in the crystal under electric stress is called the converse piezoelectric effect.
The necessary condition of the piezoelectric effect is the absence of a center of symmetry in the crystal structure. Of the 32 crystal classes, 21 lack a center of symmetry, and with the exception of one class, all of these are piezoelectric. In the crystal class of lowest symmetry, any type of stress generates an electric polarization whereas in crystals of higher symmetry, only particular types of stress can produce a piezoelectric polarization.
Piezoelectric materials are used extensively in transducers for converting a mechanical strain into an electrical signal. Such devices include microphones, phonographic pickups, vibration-sensing elements, and the like. The converse effect, in which a mechanical output is derived from an electrical signal input, is also widely used in such devices as sonic and ultrasonic transducers, headphones, loudspeakers, and cutting heads for disk recording. Both the direct and converse effects are employed in devices in which the mechanical resonance frequency of the crystal is of importance. Such devices include electric wave filters and frequency-control elements in electronic oscillators.
As described above, only non-centrosymmetric materials can exhibit piezoelectricity. Crystals sometimes have preferred directions defined by the crystalline axes and thus are often used for their piezoelectric properties. However, piezoelectric crystals are difficult to make as a thin film, because the material must be oriented. Recently, layered amorphous semiconductor structures have been synthesized which exhibit many properties similar to crystalline superlattices, including quantum carrier confinement, see e.g., B. Abeles and T. Tiedje, Phys. Rev. Lett. 51, 2003 (1983); L. Esaki and R. Tsu, IBM J. Res. and Dev. 14, 61 (1970); and H. Munekata and H. Kukimoto, Jap. J. Appl. Phys. 22, L542 (1983). These materials have conductivity, luminescence, and X-ray scattering properties which indicate that the interfaces are smooth on an atomic scale. In the present invention, it is shown that the semiconductor superlattices developed here have substantial built-in electric fields perpendicular to the layers. These fields break the symmetry of the material and make it suitable for a piezoelectric device.
A piezoelectric transducer made from amorphous superlattice materials would have the advantage that it could be deposited onto a wide variety of substrates at a relatively low temperature. Deposition onto substrates of different shapes would yield transducers sensitive to different acoustic waves. For example, a long, cylindrical transducer sensitive to long-wavelength pressure fluctuations could be made conveniently by coating a wire with piezoelectric amorphous superlattice material. This design freedom is not possible with currently available transducer materials.